1. Field of the Invention
This invention relates to the measurement of attenuation of an optical signal in an optical fiber, and is particularly concerned with a method and apparatus thereof for facilitating attenuation measurements at one or more different optical signal wavelengths.
2. Description of the Prior Art
Fiber optics have found increasing uses in many applications, especially in the telecommunication field. Fiber optic test instruments are used to characterize the quality of fiber optic links. The measure of loss (or attenuation) in a fiber optic is essential, since the loss is a limiting factor in distance communication. Moreover, measurement of a loss higher than normal, even within a loss budget of a fiber optic communication system, can pinpoint future system degradation.
Several light sources and power meters are available to measure losses in optical fibers. These instruments always require a multiple-branching procedure, when measuring the attenuation from both directions of an optical fiber. Conventional power meters will respond differently according to the wavelength of light. The user has to instruct the power meter of the wavelength actually measured. This method for compensating the response is prone to error and in most cases the user will just know the nominal wavelength of a light source, thus inaccuracy may occur as the actual wavelength of a light emitting diode (LED) or laser may differ by as much as 60 nm from each other.
Moreover, a fiber under test shows different attenuation values for different wavelengths. In order to correctly determine attenuation, it is then important to know the exact wavelength of the source used to perform the test.
* U.S. Pat. No. 4,234,253 Higginbotham et al., 18 Nov. 1980 illustrates a fiber optic attenuation measuring arrangement in which a feedback loop is used, in a transmitter, to maintain a constant output power level of a transmitted optical signal. The arrangement includes a test signal accompanying a high-amplitude timing pulse. At a receiver end, the timing pulse is separated and used to demodulate the test signal. The test signal is compared to a reference signal, to determine attenuation of a fiber under test. The invention is not concerned with measuring attenuation at different optical signal wavelengths nor with bi-directional characteristics.
* U.S. Pat. No. 4,673,291 Heckmann, 16 Jun. 1987, illustrates an optical attenuation measuring arrangement in which the light power of an optical signal input to a fiber is encoded on the optical signal, using a digital pulse frequency modulation, the optical signal input is demodulated at the receiver end and thereafter used in measuring attenuation of the optical signal in the fiber. This is an improvement since the use of digital communication enables a faster and more efficient protocol and flexibility proper to digital systems, compared to older all analog technologies. However, Heckmann is not concerned with measurements at different wavelengths.
* U.S. Pat. No. 4,726,676 Maslaney et al. 23 Feb. 1988, illustrates an optical attenuation measuring arrangement in which optical test signals of different wavelengths are modulated with respective AC signals to identify the respective wavelengths arriving at a receiver end. A comparison value, which takes into account the wavelength-dependent sensitivity of a detector of the receiver, is stored in the receiver for each optical signal wavelength and is used with a received optical signal transmitted via an optical fiber. Although this is an improvement over existing methods since the transmitter provides the exact actual wavelength to the receiving unit instead of assuming a nominal value, this arrangement requires expensive temperature stabilized circuits in order to maintain the wavelength values of optical sources. Maslaney may also transmit both power and wavelength but requires as many different modulating AC signal frequencies as there are different optical signal wavelengths and powers. The receiver scans numerous AC signal frequencies before hitting the proper frequency, thus involves a time consuming protocol.
* U.S. Pat. No. 4,737,026, Dalgoutte, 12 Apr. 1988, illustrates an apparatus that combines two transmitting optical sources and one receiving channel on a single optical fiber. This invention is a reflectometer and is not concerned with measuring optical attenuation. Bi-directional loss testers may be known as a result of combining transmitting and receiving optical ports of an invention such as described in Maslaney and including a device such as fiber optic coupler, as shown by Dalgoutte. Such a setup would still have the same limitations as the Maslaney approach and would require an efficient method in order to implement it efficiently.
* U.S. Pat. No. 5,305,078 Lamonde, 19 Apr. 1994, illustrates a system wherein attenuation of an optical signal is measured by transmitting, to the fiber, an optical signal having a pre-defined wavelength, and by FSK modulating of the optical signal with information identifying the wavelength and transmitted power of the optical signal. At the receiver end, the wavelength information is used to scale the gain of a receiving amplifier to compensate for detector gain vs. wavelength response. A DC continuous wave optical signal is then measured and converted into a digital value for use with the transmitted power information to determine the fiber attenuation at the predetermined wavelength. By means of separate transmitting and receiving ports, this invention provides easy referencing of jumper cable losses without having to join two testers together. This invention has the same limiting factor as Maslaney with respect to wavelength drift when using laser without temperature stabilization circuits as it does not provide adequate insensitivity to wavelength drift after a permanent memory factory calibration. The use of a DC continuous wave signal increases noise and subjects the invention to offset drift.